It is known that coated photographic materials, such as sensitized photographic film and print paper, are manufactured by coating extremely thin layers of material, in liquid form, onto a continuous web, sometimes termed a support. The support may be formed, for example, of a film of cellulose acetate or polyethylene terephthalate, or paper. The liquid coating is dried and the dried material is wound up prior to being unwound for, in some instances, having a further coating applied thereto, and, in other instances, cutting into strip or sheet form and for other "finishing" operations.
Sensitized photographic materials have to be manufactured to very small tolerances. The liquid material has to be applied to the web with a very high degree of thickness uniformity, both laterally and longitudinally of the web. Once the liquid material has been applied to the web with a very high degree of thickness uniformity, that uniformity has to be maintained up to the time the material is no longer fluid. In any photpgraphic manufacturing process which is commercially acceptable, the coating has to be performed at speeds as high as hundreds of feet per minute, for example, 500 feet per minute. To add to the difficulties of producing flaw free product, the viscosities of the liquid materials coated on the web are relatively low, such as, of the order of 10 to 40 cp, and hence the liquid materials flow readily under gravity or forces resulting from surface tension imbalance.
The step of drying the coated liquid material requires the application of much heat and the withdrawal of much evaporated liquid. These two requirements are best fulfilled by directing heated air at the coated layer. The heat evaporates the liquid and the air flow takes the evaporated liquid away. However, directing air flow at a liquid surface tends to deform the surface in at least two ways. Firstly, there is physical deformation due to non-uniform impingement of air and, secondly, there are random regions of surface tension differential due to uneven cooling which, in turn, is due to non-uniform impingement of air. These physical forces can deform the surface of the liquid coating to an extent which would be unacceptable in a sensitized photographic product if the deformations persisted into the product. Those skilled in the art know that if the thickness of a material in the coating varies so does the density of the photographic image created after exposure and processing of the material.
The photographic manufacturing industry has largely solved the problems associated with blowing drying air against a liquid layer on a web by causing the layer to gel or set before directing flows of drying air against the coating. The liquid materials coated onto a support usually include a binder material as well as other materials, such as silver halide, which play roles in the chemical reactions involved in the image making process. The binder material becomes a semi-rigid solid or gel when the coating is heated or cooled. In cases wherein the binder is gelatin, it is caused to set by chilling. In the set condition of the coated layer, the drying air flows can safely be directed at the coated layer without adverse effect on the thickness uniformity of the layer.
It will be understood that the chilling operation, being unlike the drying operation because there are not the large volumes of evaporated liquid to be removed, can be performed without directing air flow at the liquid coating. Chilling is often performed by directing chilled air at the uncoated side of the web and/or by passing the uncoated side of the web in contact with chilled rollers.
However, while the photographic manufacturing industry has largely solved the problems of drying a coating without the drying air itself adversely affecting the thickness uniformity of the coated layer, there is one problem which remains to be solved. This problem derives from any non-planarity there may be in the surface of the web onto which the layer of liquid material is coated. While the lack of planarity of the surface is usually acceptable from the viewpoint of its disturbing an image or object plane, it is only so acceptable if it does not adversely affect the uniformity of thickness of the coating. In other words, such lack of planarity is acceptable if the coated layer lies with uniform thickness on the surface and follows its imperfections. The height of the surface imperfections of the web being considered here are of a scale approaching microscopic and may be due, for example, to cross streaks, or the like, in cellulose acetate or polyethylene terephthalate support. For example, a frequent source of surface imperfections results from pulses in pressure in the feed of dope to the casting die when making cellulose acetate film to be used as support. Of course, every endeavor is made to produce supports with planar surfaces for receiving coatings. However, there is a practical limit to the planarity that can be achieved and to success in avoiding random occurrences which produce random defects in planarity.
The photographic industry is well able to apply liquid coatings which have uniform thickness both laterally and longitudinally of the support. However, until the liquid layer is set by chilling, there is the opportunity for forces such as gravity and surface tension to affect adversely the thickness uniformity which was achieved in the actual act of applying the liquid material to the web.
Let it be assumed that after coating with a layer of uniform thickness and before setting, the coated web is moving horizontally with the liquid coating facing upwards. If the web surface has imperfections in planarity, then, immediately after coating, the liquid-to-air surface, the free surface, of the coating exactly mimics the surface of the web immediately beneath it. Thus, there are pressure imbalances in the liquid layer due to the liquid-to-air surface not being planar. Gravity tends to reduce such imbalances in pressure in the liquid and, in so doing, causes flow which tends to level the surface of the liquid layer. There is flow away from over a high spot of the web surface towards an adjacent lower area of the web surface. Such flow causes loss of the thickness uniformity achieved at the time of application of the liquid material to the web.
There is a further physical phenomenon which is also in play, and this is surface tension. As is known, surface tension tends to oppose deformation of a liquid surface from a planar condition. Thus, with the coating liquid on top of a horizontal web which has imperfections in surface planarity, with the surface of the coating resembling the surface of the underlying web, as is the case if the coating has uniform thickness, surface tension is tending to level the surface.
Thus, in known arrangements in which the liquid coating is on the upper side of the web for chilling and setting, both surface tension and gravity are tending to work together to destroy the thickness uniformity achieved at the time of coating.
It is an object of the present invention to overcome the problems described above which are caused by flow of the liquid material on the web caused by imperfections in the planarity of the web surface.